1. Field of the Invention
The present invention relates to a method and apparatus for producing paperboard packaging blanks (trays, lids, cartons, or combinations) in which the application of the barrier coating is combined with the printing of the sales graphics in a single-pass operation which eliminates the need for a separate off-line coating operation.
2. Description of the Prior Art
To meet complex purity and performance specifications, highly specialized packaging systems have been developed for distributing, marketing and heating food for service and consumption. Many of these packaging systems are based upon a structural substrate folded from a pre-printed and die-cut bleached sulphate paperboard as described by U. S. Pat. No. 4,249,978 to T. R. Baker, entitled "Method Of Forming A Heat Resistant Carton", U.S. Pat. No. 3,788,876 to D. R. Baker et al., entitled "Carton Blanks Printed with a Heat Sealable Composition and Method Thereof" and commonly assigned U. S. Pat. No. 4,930,639 to W. R. Rigby, entitled "Ovenable Food Container with Removable Lid".
In the case of food packaging, to protect the paper package or carton from moisture degradation, due to direct contact with a food substance, the internal surfaces of such a carton are coated with a moisture barrier of one or more continuous films of thermoplastic resin. In other food and non-food applications paperboard is coated with barrier films which provide resistance to oxygen, fragrance or other gas molecule transmission. These films are usually applied to the paperboard web, prior to printing and cutting, as a hot, viscous, extruded curtain or as a viscous solution or emulsion using conventional coating techniques. Low density polyethylene (LDPE), polypropylene (PP) and polyethylene terephthalate (PET) are three of the extruded thermoplastic resins commonly used for this purpose. Acrylics, polyvinyl dichloride (PVDC), and PET are commonly applied using conventional viscous coating techniques.
Cartons for paperboard-based food packaging may take one of several forms including a top flap that is an integral continuation of the same paperboard sheet or "blank" from which the carton is erected, such a top flap being crease hinged to one sidewall of the carton. Another type of carton commonly used for food packaging has a lid independent of the paperboard blank from which the carton is formed. The lid for such a carton can be attached to the carton in various manners. Some of the common techniques include an attachment to the sidewalls of the carton or to peripheral flanges extending from the sidewalls of the carton.
However, cartons and lids of the foregoing description require two separate converting operations following the manufacture of the paperboard: 1) off-line coating or extrusion of the thermoplastic barrier coating; and 2) printing of the sales graphics. Consolidating these operations would offer obvious economic advantages.
Also, extrusion or off-line coating operations, presently used to produce the base substrate for the packaging described, are performed in a facility separate from the plant printing graphics. Inherent in having two separate operations is added transportation costs and waste. Both operations when performed separately require trim to be taken from both edges of the web, typically amounting to a substrate and coating material waste factor up to 15%. Consolidation of these operations and utilization of the coatings and techniques described herein cuts the waste by 50% or more and eliminates the need for additional transportation costs. The rolls are obviously handled less due to the reduction in transit; therefore, an added benefit of the processing described is a substantial decrease in inherent transit damage to the rolls.
Moreover, relatively high polymer coat weights are required for an extruded moisture barrier (typically from 11 to 26 pounds per 3000 ft..sup.2 ream) since lighter coat weights usually result in an inconsistent polymer layer thickness or a layer with little or no adhesiveness to the paperboard.
Finally an extruded polymer moisture barrier greatly complicates those recycling procedures necessary to recover the carton fiber constituency.
It is apparent from the above that there exists a need in the art for a method and apparatus which is capable of producing paperboard barrier packaging which avoids the high cost and waste associated with the prior, known methods and apparatus for producing barrier packaging.